Looking forward to v3.0.
Many people pass js files through custom_rhino before using packer. rhino makes JavaScript code more well formed and ensures that the packed code works. rhino already shortens variable and argument names. Maybe the new version of packer can take advantages of custom_rhino.

Nao, here is another javascript compressor. It compresses several kB better and seems to compress prototype.js ok without any changes made to it. No javascript errors popup after it is compressed.
http://www.fanskap.se/crunch

We attempted to implement this PHP JS packer into a product we are building. While the packer works perfectly in our development systems, the second we pushed it into production it totally choked. Be advised that if you plan to use this in a high traffic environment, you would be wise to stress test it before exposing it to high volumes of on-the-fly packing.